1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device for removing residual charge.
2. Discussion of the Related Art
In general, a liquid crystal display device includes a TFT substrate, which includes gate lines and data lines arranged in a matrix configuration and a thin film transistor (TFT) is formed at the intersection of the gate and data lines, a color filter substrate attached to the TFT substrate, and a liquid crystal material layer filled between the TFT and color filter substrates. In addition, a pixel electrode and a common electrode are formed with the liquid crystal material layer therebetween, wherein light transmittance of the liquid crystal material layer is controlled by supplying a voltage to both the pixel and common electrodes. Furthermore, the gate lines and the data lines are, respectively, connected to a gate driver and a data driver to receive a gate voltage and a data voltage, respectively.
FIG. 1 is a schematic cross sectional view of a liquid crystal display device according to the related art. In FIG. 1, a common voltage line 110 is formed on a first transparent substrate 100, a common electrode 130 is formed on a second transparent substrate 120, and a silver dot 140 is electrically interconnecting the common voltage line 110 and the common electrode 130. The common voltage line 110 is formed on the first transparent substrate 100 and receives a common voltage (Vcom). Accordingly, the common voltage (Vcom) is supplied to the common electrode 130 of the second transparent substrate 120 through the silver dot 140.
Although not shown, the common voltage line 110 is connected to a gate line and a data line through a static electricity preventing device. Accordingly, the static electricity preventing device is turned ON when a high voltage static charge flows into a liquid crystal display device to disperse the static charge to every gate line and data line, thereby preventing damage to the liquid crystal display device by the static charge.
In order to prevent degradation of the liquid crystal material, the liquid crystal display device employs a dot inversion method in which the liquid crystal material is operated by an AC voltage. Accordingly, a data voltage is inverted for every frame centering around the common voltage (Vcom) that is supplied to the common electrode 130. So that a data voltage with the same size as that of a data voltage supplied from an odd frame can be supplied to an even frame, a common voltage (Vcom) in consideration of a kick back effect owing to a parasitic capacitance of the TFT should be applied to the common electrode 130 of the second transparent substrate 120. Accordingly, since the kick back voltage works in the same direction as a gate voltage change, a lower common voltage (Vcom) than a central value of the inverted data voltage should be supplied. However, such voltage compensation has been made according to human vision, and thus an accurate compensation cannot be expected.
Failure to supply an accurate voltage compensation for the common voltage (Vcom) leads to accumulation of residual charge in the common voltage line 110 of the first transparent substrate 100 and in the common electrode 130 of the second transparent substrate 120. Accordingly, the residual electric charge should be discharged when the liquid crystal display device is turned OFF. However, discharging the static charge through the gate and data lines does not provide complete discharge such that even though the liquid crystal display device in an OFF state, the charge remains in the liquid crystal display device. In addition, as an overall size of the liquid crystal display device increases and image resolution improves, a line resistance increases in the liquid crystal display device, whereby the residual charge cannot be effectively removed with the discharge path.
When the residual charge is not properly discharged, a DC voltage is continuously supplied to the liquid crystal display device even if the liquid crystal display device is in an OFF state, causing a problem that an ionic contaminant (e-) is adsorbed and the TFT is degraded. Accordingly, the ionic contaminant generated due to an encapsulation material around a liquid crystal injection opening results in an abnormal stain, causing defects in the liquid crystal display device and degrading image quality.